Andrew Sarangan wrote
Except for weather

Yes, except for weather. But you know, except for weather, effective
and complete pilot training could be done in about 40 hours. Except
for weather, there would never be a need to get an instrument rating.
Except for weather, all trips could be planned in detail before
leaving. Except for weather, you could plan your flight and fly your
plan with complete confidence.

Are there other factors?

Terrain? Would not be a factor except for winds and temperature
(weather).
Traffic congestion? Could be planned for perfectly, if weather was
fully predictable.

In reality, weather is probably the biggest issue in light airplane
flying.

The longer the trip, the lower the probability of completing it
without encountering questionable weather. You can wait out the
weather on a 200 mile trip; on a 2000 mile trip you're going to have
to fly in it. That requires a higher level of skill and (if you're
dealing with any sort of time constraint) judgment. Of course there
are always exceptions - you can fly a Champ around the country,
landing in every state, and never fly in any questionable weather. In
fact, you can do the flight entirely in sunshine. It will take
months.

Weather is different in different parts of the country, but the basic
principles of mechanics and thermodynamics that underlie it are the
same everywhere. If you always fly in the same area, you learn the
specifics of that one area, and you can do that without a solid
understanding of the mechanism. You can learn by rote - red sky in
morning, sailor take warning. If you cross weather systems, you have
to learn weather at a deeper, more fundamental level - or get stuck a
lot.

Having said that, I don't necessarily agree with your other points
either. ATC is not the same everywhere. You can fly on the Gulf
Coast for years without getting a reroute in the air. On the East
Coast, I've never managed an IFR flight of more than 200 miles without
a reroute. That may well reflect my limited understanding of the
system - perhaps other people can do better - but that's only further
proof that making more long trips makes a difference.

Then there are the factors that you supposedly know about. We all
learn about density altitude and doing full-power runups, but it's a
very different experience when you take off and your rate of climb is
200 fpm. Especially when there is thermal activity. You can read
about it in a book, but it's not the same as actually being there. If
it were, experience would not count.

On a long trip, you go places that not only have you never visited,
but nobody you know has actually visited. You have to learn to handle
surprises - like that beacon that got moved half a mile, to the other
side of the runway, that all the locals know about, but which is still
depicted in the old location on the approach plate. Makes shooting an
NDB approach to mins at night a real treat.

Long trips are concentrated experience. There really is more to it
than a series of short local trips. I find it amazing that someone
who has actually done a lot of long trips would not see that.

Michael